MILAN, Nov 7 (Reuters) - Italy's fourth-largest bank Monte dei Paschi di Siena, rescued from the brink of collapse by a state bailout, returned to the black in the third quarter thanks to one-offs.

Weighed down by mismanagement, a derivatives scandal and bad debts, the world's oldest bank turned to Rome for help after failing, in late 2016, to find buyers for a 5 billion euro share issue needed to keep it afloat.

The bank, which suffered heavy deposit outflows before the bailout, said it had added 1.6 billion euros in time deposits and current accounts since the end of June, bringing the total so far this year to 11 billion euros.

It posted a third-quarter profit of 242 million euros ($280 million) thanks to a one-off gain linked to a debt-to-equity swap it carried out under European rules requiring private investors to bear losses before any state aid is allowed.

By lowering debt payments, the conversion of the bank's junior bonds into shares also helped its net interest income, which rose 5.5 percent quarter-on-quarter, though it was down 9.5 percent from a year earlier in the first nine months.

Fees fell 13.5 percent annually in January-September. The bank posted a net loss of 3 billion euros in those nine months, more than three times its loss in the same period of 2016.